Sedimentational methods are generally used for the determination of size-distribution of particles in disperse systems, particularly in suspensions. These methods utilise the Stoke's relation, the essence of which expresses that the velocity of down-streaming particles depends on their sizes. There are several techniques known in the field of sedimentational methods.
In the so-called pipette technique small samples are taken from the suspension by means of pipettes, and the solid material contents of these samples are measured. The greatest drawback of this technique lies in that the continuous sampling process diturbs the sedimenting medium.
According to aerometrical technique sinking bodies of known densities are dipped in the suspension that sink during sedimentation always deeper and deeper. Respective sinking bodies represent respective points of the particle distribution curve. The most dominant drawback of this technique lies in that the sinking bodies can not well be seen and observed in the suspension.
The photosedimentational technique is connected with a series of criteria that can not well be satisfied in the practice, because all particles should be perfectly nontransparent, there should not be light reflection between respective particles, and between particles and the wall of the test tube, and finally the suspension is sufficiently diluted not to allow that two particles can fall in the linear path of light.
In the technique of sedimentational scales, or as it is also called the gravitational cumulative method, it is a general problem that the disc of the scale is completely dipped in the suspension and it may cause a streaming therein, furthermore under the scale disc, owing to the change in density of the suspension, convectional streamings can arise that disturb the measurement.
The general disadvantages of activation analytic methods are well known, they can be carried out only in specially equipped laboratories.
The method according to the invention is directly based on the so-called pressure drop technique, in which the concentration change occuring in the sedimenting suspension is measured as a hydrodynamical pressure difference being proportional to the change in specific weight. There are two alternatives of this known technique.
The first alternative is based on the physical phenomenon according to which in communicating vessels (in the present case in an U shaped tube) the height of two fluid columns with different specific weights, measured from the boundary surface of the two fluids, varies inversely with the specific weight of the fluids.
In the second alternative the pressure difference is measured by means of differential manometer within the sedimentational tube.
The principal drawback of both alternatives of this technique lies in that the change either in the level difference or in the pressure that can be read out from the manometer is very little, whereby the visual reading of the values is difficult and inaccurate. The sedimentational and granulometrical curves should be plotted manually from the measured data. The practical performance of the measurements is difficult which has practically prevented this technique from being widely accepted.